Automotive leak detection systems can use either positive or negative pressure differentials, relative to atmosphere, to check for a leak. Pressure change over a given period of time is monitored and correction is made for pressure changes resulting from gasoline fuel vapor.
It has been established that the ability of a leak detection system to successfully indicate a small leak in a large volume is directly dependent on the stability or conditioning of the tank and its contents. Reliable leak detection can be achieved only when the system is stable. The following conditions are required:
a) Uniform pressure throughout the system being leak-checked;
b) No fuel movement in the gas tank (which may results in pressure fluctuations); and
c) No change in volume resulting from flexure of the gas tank or other factors.
Conditions a), b), and c) can be stabilized by holding the system being leak-checked at a fixed pressure level for a sufficient period of time and measuring the decay in pressure from this level in order to detect a leak and establish its size.